Method of producing current conducting elements



June 1965 c. E. RANSLEY 3,186,044

METHOD OF PRODUCING CURRENT CONDUCTING ELEMENTS Original Filed May 23, 1957 3 Sheets-Sheet 1 Q 2 E R & Q q 8 2 k m a k E 2 Q E h E. w:

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9 4 z CL/ Q g g g g: g; 5 INVENTOR ATTORNEY METHOD OF PRODUCING CURRENT CONDUCTING ELEMENTS- Original Filed May 23, 1957 June 1, 1965 c. E. RANSLEY 3 Sheets-Sheet 2 INVENTOR Ljmrlea Eric mslqy BYW 7 ATTORNEY June 1, 1965 Original Filed May 23, 1957 c. E. RANSLEY 3,186,044

METHOD OF PRODUCING CURRENT CONDUCTING ELEMENTS I5 Sheets-Sheet 3 ATTORNEY United States Patent 3,186,044 T1101) 0F PRODUCWG C 'r. CUNDUCTHNG ELEMENTS Charles Eric Ransley, Cliesham, Bois, England, assignor to The British Aluminium Company Limited, London, England, a company of Great Britain Original application May 23, 1957, Ser. No. 660,994, now Patent No. 3,028,324, dated Apr. 3, 1962. Divided and this application June 12, 1961, Ser. No. 125,317 Claims priority, application Great Britain, May 4, 1951, 10,548/51, 10,549/51 17 Qlaims. (Cl. 22--202) This invention relates to electrolytic cells for the production of metals, e.g. aluminum. The expression electrolytic cell, as used hereinafter, is meant to include both reduction cells for the production of aluminum and three-layer cells for the refining or purification of aluminum. More particularly this invention is concerned with electrolytic cell-s embodying one or more current-conducting elements Which may constitute at least a part of the cathodes of reduction cells or current-conducting elements for taking part in the supply of electrolyzing current to a body of molten metal and at least partially exposed to the latter either in a reduction cell or in a purification cell. Additionally, this invention is concerned with electrolytic cell structure involving the use of such current-conducting elements and methods of operation of such cells.

This application is a continuation-in-part of my copending applications Serial Number 286,709, filed May 8, 1952; Serial Number 481,611, filed January 13, 1955; Serial Number 481,927, filed January 14, 1955; Serial Number 569,736, filed March 6, 1956; Serial Number 569,737, filed March 6, 1956; Serial Number 570,233, filed March 8, 1956 and Serial Number 613,006, filed October 1, 1956, said application Serial Number 613,006 being a continuation-in-part of said Serial Numbers 286,709; 481,611; 481,927; and 569,737 and application Serial Number 284,761, filed April 28, 1952 (now abandoned). This is a divisional of application S.N. 660,994,

filed May 23, 1957, now Patent No. 3,028,324 granted April 3, 1962.

Both of the above-mentioned cells, that is, the reduction cell and the three-layer cell are of the kind in which it is necessary to pass electrolyzing current through a body of electrolyte or flux. In the case of reduction cells the current passes between an anode and a cathode having their operative faces in contact with the body of electrolyte which has dissolved therein a compound of the metal. The cathode may be the pool of molten metal which collects on the floor of the cell or it may be a solid electrode immersed at least partially in the electrolyte. Such an electrode may extend into the pool of molten metal in which case the latter is also cathodic.

. In the case of three-layer purification cells the current passes between the pool of aluminum alloy forming the bottom layer in the cell and the layer of purified molten aluminum forming the top layer in such a cell through the body of electrolyte or flux forming the intermediate layer which is in contact with both the top and bottom layers. Hitherto no material has been found to have the chemical and electrical properties required of a solid electrode constituting the cathode in a reduction cell and further the arrangements at present made for leading the current into the body of molten metal in a reduction cell or the bodies of metal in a three-layer cell are not entirely satisfactory and this is particularly true of the reduction cell where relatively substantial losses in etliciency and increases in constructional and maintenance costs are directly traceable to the nature of the current leads which have to be employed.

3,186,044 Patented June 1, 1965 For example, aluminum reduction cells which are at present in commercial use are employed to effect the electrolysis of an aluminum compound, generally aluminum oxide, while it is dissolved in a suitable flux which is mainly cryolite and has a fusion point usually in excess of 900 C. Since the cells therefore must be operated at a temperature in the neighborhood of 1000 C., their construction has always presented considerable problems. The flux, for example, is very reactive towards metals and towards normal refractory materials. Thus difficulty is experienced in constructing a durable receptacle or container for the molten flux and there is even greater difiiculty in finding suitable materials for the construction of a solid cathodic electrode in contact with molten flux and molten metal and the current-conducting elements which are in contact with the molten metal.

Carbon is the only material which has hithereto been found to be capable of use for the purposes mentioned above, this being employed both for lining the receptacle which is to contain the electrolyte or flux and for the construction of the current-conducting elements. However, the use of carbon entails a number of very considerable disadvantages, not the least of Which is the fact that the floor of the cell lining which supports the molten metal must, in practice, be arranged in a substantially horizontal plane. With such arrangement the floor space occupied by a single cell is quite extensive and the cost of constructing such large cells is considerable. The necessity for this horizontal arrangement arises from the fact that molten aluminum does not wet carbon. Unless this arrangement be adopted the current efliciency of the cell is very low.

In the case of reduction cells, the use of carbon to conduct current to the molten metal cathode and the horizontal arrangement of the cell floor entail a number of disadvantages in operating the cell. For example, the gradual penetration of molten flux or flux constituents into the cell floor causes this to disintegrate and shortens its useful life. Deposits are formed on the surface of the carbon which increase the voltage drop across the cell and reduce the efliciency of the latter. Satisfactory electrical contact between the carbon lining and the electrical current supply conductors, e.g. cathode collector bars, is difiicult to achieve and there are appreciable losses also due to the electrical resistance of the carbon itself.

The horizontal construction referred to hereinabove has the further disadvantage that the inherent turbulence of the molten metal cathode requires a high inter-polar distance to ensure against contact of the molten metal cathode with the anode and with the consequent production of excess heat which has to be dissipated.

Accordingly, it is the primary purpose and object of this invention to provide improved electrolytic cells for the production of aluminum, e.g. reduction cells and refining or purification cells, which overcome or substantially minimize the disadvantages as mentioned hereinabove.

Another object is to provide novel current-conductin elements for use in electrolytic cells.

Another object is to provide methods for making current-conducting elements.

Another object is to provide improved reduction and refining cells for the production of aluminum wherein the cathodic electrical system does not involve, as an element therein, a carbon or graphite member.

These and other objects and advantages of this invention will be apparent from the following description thereof taken in conjunction with the drawings wherein:

FIGURE 1 is a diagram showing the amount of titanium dissolved by aluminum at 970 C. in contact With s eep a4.

'3 .9 materials ranging in composition from 100% TiC to 100% TiB FIGURE 2 is a vertical section of a three-layer refining or purification cell;

FIGURE 3 is a vertical section of a three-layer refining or purification cell showing an alternative arrangement of the current leads supplying the top layer;

FIGURE 4 is an elevational view, partially broken away, showing a sheathed electrode in its position of use;

the disadvantages in prior art cells and cell operation,

as mentioned hereinbefore, there has been a long sought need for a new current-conducting element which may be exposed in part to molten aluminum, molten aluminum-containing metals, electrolyte or flux and temperatures and atmospheres incident to the operation of elec trolytic cells Without material adverse effect. It has been concluded that the properties required in an ideal current-conducting element may be summarized as follows:

( 1) It should have a good electrical conductivity.

(2) It must not react with nor be soluble in either molten aluminum or, under cathodic conditions, in molten flux or electrolyte, at least to any appreciable extent, at the operating temperature of the cell. The solubility of the material in molten aluminum is an important consideration as it determines both the useful life of the current-conducting element and the degree of contamina tion of the aluminum produced through the agency of such current-conducting element.

(3) It should be capable of being wetted by molten aluminum. The importance of wettability had not previously been recognized clearly, but my research demonstrated that immediate advantage would follow if a material with this property could be developed.

(4) It must be cheap enough to be fabricated in the required forms economically.

(5) It should have high stability under the conditions existing at the cathode of the cell, that is, it should possess good resistance to penetration by the molten metal and to cracking.

(6) It should have a low thermal conductivity.

(7) It should have a good mechanical strength and resistance to thermal shock.

(8) Wherethe material of the current-conducting element is to be exposed to the exterior of the cell, it should have a good resistance to oxidation and to gases to which it is exposed. Normally the conditions existing at the cathode of a cell are highly reducing and in certain applications therefor this requirement is not essential.

As the result of many experiments, it has been found that materials which exhibit all or substantially all of the properties hereinabove set forth are the carbides and borides of titanium, zirconium, tantalum and niobium and mixtures thereof.

At least the operative face or faces of the currentconducting elements, i.e. the face or faces exposed to the deleterious conditions subsisting during the operation of the cell, e.g. the face or faces exposed to the molten metal, may consist essentially of but one of the materials specified, or, alternatively, may consist essentially of more than one such material in varying proportions. In most applications, it is preferred that the whole of the current-conducting elements should consist essentially of one or more of such materials. The expression consist essentially, as used hereinafter in the specification and claims, means that that portion of the element made of oneor moreof the carbides and borides referred to above does not contain the substances in amounts sufficient materially to affect the desirable characteristics of the current-conducting element although other substances may be present in minor amounts which do not materially affect such desirable characteristics, for example small proportions of oxygen or iron in titanium boride. It is also preferred that the refractory materials in the current-conducting elements be essentially free from elements or compounds which would lead to undesirable contamination of the aluminum produced. Nevertheless, the current-conducting elements embodied in cells according to this invention may contain initially, among others, certain compounds which dissolve out when the element is first put into service, and which do not materially alfect the element.

Desirably, that portion of the element consisting essentially of one or more of the above mentioned refractory materials should be composed of at least by weight of such material. The carbides and borides referred to have been found to possess a relatively high electrical conductivity (better than that of carbon), a good resistance to attack by molten flux or electrolyte, a very low solubility in molten aluminum at cell operating temperature and a resistance to oxidation considerably better than that of carbon. They can be produced in a suitable form with good mechanical properties. Furthermore, it is possible effectively to wet the surface of current-conducting elements made from these materials with molten aluminum under all operating conditions, from which it results that, for the first time, a commercially practicable cell with vertical or inclined electrodes may be constructed. The term cathode, as applied to solid members, is intended to denote an electrode of sheet, plate, rod or other suitable shape at which metal is produced in a tangible form. In addition, these materials can be connected without great difiiculty to a metallic conductor to establish a good mechanical and electrical joint therewith.

Of the carbides and borides referred to above, those preferred at the present time are the compounds of titanium and zirconium since the elements tantalum and niobium are relatively rare and correspondingly expensive.

Accordingly, the present invention provides in an electrolytic cell for the production of aluminum the combination comprising a receptacle defining a chamber adapted to contain a body of molten electrolyte and a body of molten aluminum and an electrical system including at least one anodic current-conducting element extending into said chamber and at least one cathodic current-conducting element extending into said chamber, at least a part of the surface of one of said current-conducting elements being adapted to be exposed to molten aluminum and consisting essentially of a material possessing a low electrical resistivity, a low solubility in molten aluminum and molten electrolyte under cell operating conditions and being wettable by molten aluminum under cell operating conditions.

According to a further feature of the invention an electrolytic cell for the production of aluminum comprises in combination a receptacle defining a chamber adapted to contain a body of molten electrolyte and a body of molten aluminum, and anode extending at least partially within said chamber and a cathodic current-conducting element having at least a part of its surface exposed to the interior of said chamber and spaced from said anode, at least said part of said surface consisting essentially of a material possessing a low electrical resistivity, a low solubility in molten aluminum and molten electrolyte under cell operating conditions and being wettable by molten aluminum under cell operating conditions.

It is also a feature of the invention that said material referred to in the preceding two paragraphs has an electrical resistivity lower than carbon and a solubility in molten aluminum and molten electrolyte under cell operating conditions at least aslow as titanium carbide or zirconium carbide.

Advantageously said material is titanium boride.

A current-conducting element consisting essentially of one or more of carbides and borides referred to may be employed with advantage in accordance with this invention to establish electrical connection with the body of molten aluminum-containing metal in a cell of orthodox construction, its use greatly reducing the voltage drop which would otherwise be experienced. Additionally, as mentioned above, such a current-conducting element may be used satisfactorily as a cathode disposed in a vertical or inclined position. There are considerable advantages to be gained by constructing a cell with a cathode or cathodes at least a part of the operative surface of which is so disposed. Current-conducting elements for use in electrolytic cells according to this invention can be produced from the carbides and borides referred to having a reasonably low electrical resistivity, i.e. in the range of from about to 120 microhms cm., a low solubility in molten aluminum, i.e. not more than about 0.04% in molten aluminum at about 970 C., a good resistance to attack by the molten electrolyte employed in electrolytic cells for the production and refining of aluminum and a resistance to thermal shock such that the elements will withstand plunging into molten aluminum at 750 C. at a temperature differential not less than 200 C. without cracking. They are thus eminently suitable for use as leads for taking part in the supply of electrolyzing current to a body of molten aluminum in such a cell or as cathodes (or facings for cathodes) in electrolytic reduction cells for the production of aluminum.

With regard to the carbides referred to above, titanium carbide is preferred to zirconium carbide for the purposes in view, not only because it is less expensive to produce but because it has a much higher resistance to oxidation than zirconium carbide. When the latter is employed, in fact, precautions must be taken to insure that it is never exposed to the action of air or oxygen or oxidizing conditions while at a high temperature, e.g. the operating temperature of the cell, for which reason a cathode or current-conducting element composed entirely or consisting essentially of zirconium carbide should be protected, e.g. by protecting it with a sheath of oxidation resistant material before its temperature is raised to any substantial degree. In addition, zirconium carbide requires higher temperatures than does titanium carbide for the carrying out of the method of producing the coherent mass of the carbide having the requisite mechanical strength. For these reasons, the following discussions with regard to carbides will be concerned mainly with the use of titanium carbides but the several steps and procedures detailed in this connection apply also in case ofthe other carbides, as well as in the case of the borides, save where a note is given of the necessary modification or where the necessity for protecting the other carbides against oxidation will entail corresponding precautionary measures.

The current-conducting elements, e.g. cathodes, current leads, etc., are preferably prepared by providing the materials in the form of powders of suitable purity and particle size and then subjecting the material to compacting and sintering operations by hot pressing which comprises subjecting the powder, e.g. titanium carbide, to a continuously applied pressure of from about 0.5 to 50 tons per square inch, e.g. 1 ton p.s.i., while raising its temperature to about 1600 to 2700 C., e.g., 2000 C. Preferably the compacting and heating operations are carried out in a protective atmosphere, e.g. hydrogen or in a vacuum. It is preferable to raise the temperature to the maximum value in a relatively short period of time, for example, in less than about 1 hour. The operation may be carried out in a graphite die having a cavity of the appropriate crosssectional shape, the pressure preferably being applied to the powder by plungers acting on opposite ends of the column of powder and wherein the protective atmosphere is maintained around the die during the heating and cool- 'i ng periods. Although the above method has been found quite satisfactory in making current-conducting elements, it is contemplated Within the scope of the instant invention that other methods may be used, for example, fusing the materials in a high temperature furnace and casting same to desired shape or the use of cold pressing and sintering techniques.

The cold-pressing method involves the cold pressing of the powders, followed by a sintering operation carried out at a temperature between 1100 C. and 2200 C. either in vacuum or in a neutral atmosphere. For example, the material of the element, e.g. titanium carbide, may be provided in particulate or powder form having a mean particle diameter of about 1 to 2 microns with which has been mixed a small portion of a binder, e.g. 1% of parafiin wax dissolved in benzene. The benzene is evaporated otl on a water bath, or at a temperature sufficiently high to melt the wax, prior to the compacting operation. The compacting step may be effected by single pressures between male and female dies, either at ordinary temperature or an elevated temperature, and wherein the pressure applied is in the range 0.5 to 50 tons per square inch, e.g. 3 tons per square inch. Alternatively, the powdered mixture may be extruded into the desired shape. If the initial powder has a sufficiently low content of free carbon, it may be directly compacted as above and then pre-fired in vacuum to a temperature of 1100 C. (about 1600 C. for zirconium carbide). It may then be worked by sawing, filing, or like shaping operations to produce an element of appropriate form, although it will be understood that the electrode or element will usually be finally shaped in the compacting operation. The electrode or current-conducting element is then fired in vacuum at an elevated temperature, e.g. where titanium carbide is the material of the element the firing temperature can be about 1600 C. (about 2200 C. for zirconium carbide) although this may be varied according to the density desired in the final product, to produce a robust, sintered element having a porosity of the order of 20% by volume. Such an element is a selfbonded element, i.e. the titanium carbide particles adhere directly to each other, and at such a relatively high porosity the pores are interconnected and capillary paths exist Within the element so that the latter may be considered to be permeable.

Generally, the titanium carbide powder of commerce contains about of titanium carbide and 1 to 2% of free carbon, the balance being titanium oxide and titanium nitride in solid solution in the titanium carbide andcombined iron. If a powder of this character be treated as above set forth, that is, by cold pressing and sintering under pressures and temperatures such that the element is permeable, the current-conducting element obtained is not always suitable for the purposes intended because the content of free-carbon is not necessarily reduced to a safe level during a sintering operation carried out at a temperature of 1600 C. in vacuum. The use of a higher temperature either in vacuum or in a furnace in which an atmosphere of a neutral gas, such as hydrogen, is

maintained, results in improved products but some of these may still have a content of free carbon amounting to 0.6% by weight. It is found that a sintered titanium carbide compact having a porosity such that the element produced is permeable and which contains more than about 0.5% free carbon shatters or disintegrates when wetted by molten aluminum, probably due to penetration of molten aluminum up the capillary paths and internal reaction between the free carbon and aluminum to form aluminum carbide. Such an element would not be suitable for use as a cathode or current lead exposed to molten aluminum in an electrolytic reduction cell for the production of aluminum. However, the difficulty can be overcome by incorporating into the titanium carbide powder being used in the production of the element a suitable proportion of powdered calcined alumina.

Preferably, the alumina is added to the commercial titanium carbide powder and the mixture then ball-milled for a relatively long period in the dry state until it is reduced to a mean particle diameter of about 1 to 2 microns. The amount of alumina added depends to some extent upon the content of free carbon in the titanium carbide powder but is usually equivalent to about 2 to 3% of the weight of the titanium carbide employed.

The finely powdered mixture of titanium carbide and calcined alumina is then moistened with a suitable binder, e.g. parafiin wax dissolved in benzene, and the solvent driven off prior to compacting the mass under pressure as set forth above. The firing of the compact obtained is effected in a furnace through which a stream of protective gas, e.g. hydrogen, is passed, the temperature preferably being higher than 1600 C., for example, about 2200 C. (about 2700 C. for zirconium carbide). The finished product contains substantially no free carbon and the residual aluminum therein, whether present as A1 or Al C is very small. As an example, a titanium carbide powder initially containing between 1% and 2% by weight of free carbon, when mixed with 2.5% by weight of calcined alumina and treated as above set forth, yielded an element containing only 0.2% by weight of free carbon. Increasing the amount of A1 0 added to 3% resulted in an element containing 0.05% by weight of free carbon.

It is preferred that the content of free carbon in the finished element shall be below 0.1% but somewhat higher percentages of free carbon can be tolerated provided that the figure of 0.5% not be exceeded. Currentconducting elements thus prepared by the cold-pressed method from substantially carbon-free titanium carbide will be wetted freely by molten aluminum under cell operating conditions without any tendency to shatter or break up and without any sign of cracks developing.

Current-conducting elements made by the use of coldpressing techniques, at the temperature and pressure mentioned above, possess the disadvantage of having a relatively high porosity, e.g. up to 20%, and of being permeable so that the elements can be penetrated by undesirable substances, e.g. flux or flux constituents. This disadvantage of flux penetration can, however, be overcome by wetting and thoroughly impregnating the elements with aluminum at elevated temperatures, e.g. 1100 to 1150 C., in vacuum. The aluminum completely wets all the exposed surfaces of the elements and adheres thereto as a thin film. The aluminum will penetrate into any pores of the elements and impregnate them to a degree dependent upon the porosity thereof. The coating or impregnation, for example, of a TiC compact, with aluminum improves the'electrical conductivity of the compact and it increases its resistance to oxidation at high temperatures. However, as the electrical conductivity of TiC is adequate in itself the improvement in this direction, though useful, is not of great importance. On the other hand, the increase in the oxidation resistance obtained in the case of TiC is an advantage. Such porous impregnated elements should not, however, be employed in positions where they are exposed to oxidizing atmospheres at temperatures above the melting point of aluminum as there appears to be a tendency for caustic attack of the elements possibly due to sodium, a product of the electrolysis, penetrating the elements via the aluminumfilled capillary passages and then oxidizing to caustic soda.

Permeable current-conducting elements formed from the cold-pressed material are desirably pre-wetted and impregnated with aluminum before being incorporated in an electrolytic cell as the wetting and impregnation of the elements cannot be satisfactorily achieved at the normal operating temperatures of the cell. A current-conducting element made by the cold-pressed method described above, i.e. by subjecting powdered titanium carbide first to pressure and subsequently sintering the compact at a high temperature was found to have a porosity of the order of 20% by volume, i.e. a density of 3.95 compared with the theoretical density of 4.93, and a free carbon content of less than 0.5%. It had an electrical resistivity of 54 microhm cm. and, when impregnated with aluminum, an electrical resistivity of 51 microhm cm. It had a transverse rupture modulus of 6-l0 tons/ sq. in. and a thermal shock resistance to a temperature differential of from 200 C. to 300 C. Thermal shock re sistance was measured by plunging a test bar (e.g. in. diameter and 4 in. long) into molten aluminum at 750 C. The test was made quantitive by heating the bar to various temperatures before immersion and expressing the quality of the rod as the minimum temperature difference between it and the molten aluminum which would crack the bar.

The oxidation characteristics of the current-conducting element impregnated with aluminum expressed as weight increments in air at 1000 C. was as follows:

After 18 hours 5.9 10 g./cm. After 63 hours 13.9 10- g./cm.

The titanium carbide content of the material employed was found to vary from 91% to 96% and the material had a titanium nitride content of up to 5%. Iron was found to be present in amounts up to about 1%, but no particular effects are ascribed to it.

Current-conducting elements made by the hot-pressed method described above, i.e. by subjecting the powdered material simultaneously to heat and pressure, were found to be less porous than those produced by the cold-pressed method. For example, elements hot-pressed at 2000 C. under a pressure of 1 ton per square inch were found to have a porosity of less than 10% and such elements are normally impermeable. Such elements had a density of 4.4 as compared with the theoretical density of 4.93. Such elements made from hot-pressed titanium carbide can be wetted with aluminum by vacuum treatment in the same way as the more porous material referred to above, but they cannot be impregnated with aluminum as the pores in the elements are not interconnected. However, in this case such is not essential as wetting takes place automatically in the cell. Also, with hot-pressed elements the free carbon content of the material is not as critical and a dense material containing as much as 1% free carbon has been found not to crack on wetting with molten aluminum. The reason for this is believed to be that with lower densities the free carbon is dispersed in the carbide as isolated particles of second phase and local reaction with molten aluminum occurs only on the surface of the element.

The electrical resistivity of the hot-pressed titanium carbide elements was found to vary with the composition of the material. Thus with elements prepared from mineral rutile the electrical resistivity varied substantially linearly from 84 microhm cm. at by weight of titanium carbide to 63 microhm cm. at 96% by weight of titanium carbide. The values were, however, markedly lower when elements were prepared from pigment titanium oxide. This is ascribed partially to the lower zirconium and vanadium contents of the pigment oxide and also to a lower oxygen content of the material. However, a resistivity of 68 microhm cm. and a linear temperature coefficient up to 1000 C. of about 0.0008/ C. is considered to be reasonable and may be readily attained.

The hot-pressed titanium carbide elements were found to have a transverse rupture modulus of 15-20 tons/sq. in., a thermal conductivity of about 0.07 c.g.s. and a thermal expansion over the temperature range of 20 C.- 400 C. of 7.2 l0 cm./cm./ C. and over the temperature range of 20 C.1000 C. of 8.l l0- cm./cm./ C. The elements were further found to have a thermal shock resistance to a temperature differential in excess of 300 C.

Thehot-pressed titanium carbide elements were found to contain upwards of 90% by weight of titanium carbide (usually not more than 96% by Weight), up to of titanium nitride and a free carbon content and iron content of up to 1% by weight each. The .presence of the nitride, free carbon and iron had no apparent deleterious effect on the hot-pressed impermeable titanium carbide elements.

Although the carbides have a good resistance to aerial oxidation relative to carbon suitable protection should desirably be provided Where these materials are used as current-conducting elements in electrolytic cells in positions in which they are exposed to such oxidation. They are advantageously provided with a sheath as will hereinafter be described. Titanium carbide is subject to a penetrating form of oxidation at a critical temperature of about 450 C. and this type of attack at this lower temperature is thought to be due to the formation of a disintegrated and non-protective metal oxide. The oxidation resistance of this material, however, increases at temperatures above 450 C. to a maximum value at about 700 C.750 C. At still higher temperatures its oxidation resistance decreases progressively.

With regard to the effect of impurity content in the use of titanium carbide it has been found that oxygen tends to adversely affect the solubility of titanium carbide in molten aluminum in the temperature range normally found in operation of the cell, e.g. 950 C. to 1050 C. for a reduction cell. The oxygen present is in solution in the titanium carbide and probably is present in the formof titanium monoxide. It has been found that there is a strong tendency for the carbide to disintegrate Where oxygen is present above about 1% by weight. A reasonably good commercial product contains about not greater than about 0.5 and it should desirably be pre-wetted and impregnated with aluminum prior to its incorporation in an electrolytic cell. An element which is substantially impermeable, i.e. having pores which are not interconnected, e.g. one made by the hot-pressing method and having a porosity not greater than about by volume, does not have the limitation as to its free carbon con-tent and need not be pre-wet-ted with aluminum. It may, however, advantageously be pre-wetted as I such pro-wetting is a useful indication of the satisfactory If the element stands up to the nature of the material. wetting step without cracking, it isreasonably certain that the element will prove satisfactory in service and not disintegrate in the cell. Finally, the pre-wetting'of the element with aluminum facilitates the making of the preferred form of connection between the element and the external current-supply bus-bars feeding the cell. This preferred form of connection is a bar of aluminum which .is cast onto the element at its one end to be in intimate electrical contact with the carbide.

The product obtained by any of the methods set forth above is a shaped current-conducting element, e.g. a cathode, which can be eifectively wetted with aluminum. When an element has been so wetted, a bar of pure aluminum to serve as an electrical conductor may readily be fused directly thereto. Consequently, when the cathode is in service, it may be connected to a source of'electrolyzing current by a bus-bar directly fused or cast onto one end of the cathode which is not exposed to the interior of the cell, that portionof. the cathode exposed to the interior of the cell being constituted by the refractory 0.5% by weight of oxygen and this has been found to l material coated with molten aluminum. lit will be appool in the lower part of the cell, this pool preferably being in contact with the cathode. It should be noted,

"however, that the cathode remains completely Wetted by molten aluminum which adheres tenaciously thereto.

A cell having an inclined cathode constructed in accordance with the invention may have anodes of carbon constructed and fed in any suitable manner and will operate on a voltage which is considerably less than that required in cells of comparable size employing cathodes of the orthodox type. Moreover, the current efiiciency is higher than it has been possible to achieve in practice with cells of the usual construction.

With further reference to the refractory materials referred to above special reference should be made to titanium boride (TiB and zirconium boride (ZrB which have similar properties to each other, these properties being superior to those of the carbides for the purposes in view and not hitherto known. Titanium borideis readily wetted by molten aluminum under cell operating conditions and has a much lower electrical resistivity than 'bide over thetemperature range 300-750 C. (this being very marked at temperatures of about 450 C.) and at temperatures above about 850 C. due to the normal formation of a glassy oxidation phase; and has a solubility in molten aluminum at temperatures of the order of 970 C. which is only about one-tenth of that of titanium carbide. It will thus be seen that the boride of titanium is preferred to the carbide for the purposes in view.

However, all the materials referred to have electrical resistivities which are sulhciently low to make them suitable for use in the production of current-conducting elements for employment in electrolytic cells for the production of aluminum.

The lower electrical resistivity of the borides (particularly titanium boride) is of practical importance as it enables an economy to be made in the cross-section of the current-conducting elements and/ or in the number of such elements required and this helps to off-set the greater cost of these materials.

In addition to the characteristics mentioned above, the

'borides, and particularly titanium boride (T-iB and zirconium boride (ZrB have other characteristics which render them particularly suitable for the purposes in View. The free carbon content of titanium boride has not been found to be in any way limiting. Titanium boride as normally manufactured does not have a high oxygen con tent but titanium boride is not sensitive to this oxygen content at least to the same degree as titanium carbide. Thus, impermeable current-conducting elements formed from hot-pressed titanium boride having an oxygen content of 1.4% have operated satisfactorily in reduction cell tests. The purity of the material employed does not appear to be critical. For example, tests have been made with titanium boride materials having quite high impurity contents, e.g. up to 1% by weight each of free carbon and nitrogen, excess boron, carbon and iron in combined forms. Current-conducting elements composed of these materials, hot-pressed, revealed no apparent deleterious effect as regards cracking or penetration and dissolved very slowly and uniformly in molten aluminum. The purity of the material does, however, have an effect upon its electrical resistivity and can Vary it by a factor of about 4 at room temperature, i.e. from 10 to 40 microhm .cm., so that it is desirable to control the purity from this point of View.

As mentioned .above, the solubility characteristics of 1 l the borides is very much more favorable than the carbides. Thus in the reaction TiB Ti (dissolved)+2l3 (dissolved) the stoichiometric solution of the compound at about 970 leads to titanium and boron contamination in the aluminum of about 0.004% and 0.0015 respectively, i.e. about one-tenth of the titanium content observed with the carbide. This advantage of the borides, particularly titanium boride and zirconium boride, was quite unexpected and could not have been predicted.

A further advantage of the borides is that, for example, in the equilibrium solution of TiB in molten aluminum both titanium and boron are below their saturated solubilities and throughout a wide temperature range the simple solubility product law (Ti) (B) =constant; defines the conditions for the solution or precipitation of titanium boride. The dissolution of the'elements can thus be hindered by the addition of titanium, or very much more etfectively of boron, to the molten metal in the cell.

A certain amount of titanium is normally present in the metal from other sources and this prolongs the life of the current-conducting elements to some extent. The useful life of current-conducting elements composed of the borides is, however, quite long enough for the purposes in view so that normally it is not necessary to increase it by the addition of titanium or boron to the molten aluminum. However, if it is desired to increase the life of the elements composed of one or more of the borides, then boron may be added to the molten aluminum in the proportion of from 0.001% to 0.003% by weight or alternatively the metal of the boride, e.g. titanium, may be added in the proportion of from 0.001% to 0.005% by Weight. It will be appreciated that, as a further alternative, a small proportion of both boron and the metal of the boride may be added to the molten aluminum.

As an example, tests have been carried out with currentconducting elements produced from hot-pressed titanium boride. The density of these elements was found to vary from 4.1 to 4.3 (theoretical density 4.52) and the porosity of the elements was always less than The elements had a thermal expansion over the temperature range of 20 C.400 C. of 6.9 10- cm./cm./ C. and over the range of 20 C.l000 C. of 8.0)(10' cm./cm./ C. The electrical resistivity of these elements varied from 10 to 40 microhm cm. at 20 C. Elements having a resistivity of between 10 and 20 microhm cm. at room temperature were shown to have a linear resistivity-temperature relationship and a coeflicient of 0.003/ C. The elements were shown to have a transverse rupture modulus of tons/ sq. in. The elements had a thermal shock resistance to a temperature dilferential of 350 C.

The elements had an oxidation resistance which was better than those produced from titanium carbide. This is most marked at 450 C. at which temperature the titanium boride elements showed a depth of penetration of 8 microns after 120 hours whereas the titanium carbide elements showed a depth of penetration of 88 microns.

I have determined the approximate value of the solubility product of titanium and boron in solution in molten aluminum at ditferent temperatures and the following table sets out some examples of this product and the corresponding titanium concentration which is therefor observed when titanium boride (TiB dissolves in the aluminum in its correct stoichiometric proportions.

Temperature, 'Ii for stoiehio- C. (Ti) (B) 2 metric solution (percent) Titanium boride materials may in practice contain a small excess of boron which dissolves preferentially in the molten aluminum and so reduces the solubility of the titanium in accordance with the above relationship. This effect, however, is usually transient.

As has been mentioned above, the purity of the boride material employed had no appreciable eflect on the penetration of the elements. Mention should, however, be made of the fact that materials having 10% by weight of boron carbide deliberately added thereto were found to be subject to penetration effects.

Tests on current-conducting elements composed of hotpressed zirconium boride showed this material to be comparable with titanium boride.

As a further feature of the invention it has been found that special advantages can be secured by the use of mixtures of titanium carbide and titanium boride, which would not be anticipated from a study of their properties determined separately.

The mixture may contain various proportions of the two ingredients, depending upon the properties required in the final product, and in some cases, the current-conducting element may be composed of a mixture the composition of which progressively changes along the length of the element.

The most surprising effect obtained by admixing titanium boride with titanium carbide, when preparing the current-conducting elements, is the decrease in the solubility of the titanium carbide in aluminum at high temperature (e.g. at 970 C.) as shown in FIGURE 1 of the drawings. Quite small percentage additions of the boride appreciably suppress the solubility of the carbide. As will be seen from FIGURE 1, the solubility of a titanium carbide sample decreased to approximately one-third by the addition of only 10% of titanium boride and that it then remained substantially constant up to at least 70% of the boride.

Since titanium boride is an expensive material compared with titanium carbide, it is of great value to be able to secure substantial improvements in solubility by the incorporation of relatively small portions of the boride .in the cheaper and more readily available carbide. Generally, additions of the order of 5 to 25% by weight are adequate, the preferred amount being in the range from about 10 to 20% by weight. Additions of less than 10% will produce beneficial results, and it will then be a question of balancing the increased cost of the boridecontaining material against the longer life of the currentconducting elements which would be obtained under cell conditions although it is preferred to add not less than 5%.

As far as other properties are concerned, the titanium carbide-titanium boride mixtures appear to be intermediate between the carbide and boride. Thus the resistance of the mixture to oxidation in air at high temperaturcs is better than that of titanium carbide alone. The electrical resistivity does not deviate very markedly from a linear relationship with composition and decreases from a value between and microhm cm. for titanium carbide to approximately 10 microhm cm. for titanium boride. There is thus an additional advantage to be gained from the use of the mixtures, which offsets the higher cost of the boride, in that an element of smaller cross-section can be used to carry a given current.

The carbide-boride mixtures may be prepared by several methods as hereinbefore discussed. It should be noted that it is now possible to produce a current-conducting element for use in either reduction or three-layer cells which is complex in composition. Thus it is possible, for

example, to produce a bar element which contains 20% of titanium boride and of titanium carbide over only a portion of its length, the remainder of it consisting essentially of titanium carbide only. Intermediately between the two links of different composition, the proportions of the ingredients may, if so desired, be changed progressively so that a graded joint is obtained. By

l3 this procedure, the risk of cracking at the juncture between the two links of differentcompositions is minimized. Also, it is feasible to produce current-conducting elements which consist of a mixture of carbide and boride where they protrude into the molten cathode pool of a cell, and of carbide only where they are buried in the wall or bottom of the cell itself.

It is further feasible to produce current-conducting elements which consist of the borides only where they protrude into the molten metal,the remainder of the element consisting of the carbide only. This is possible as the coefiicient of expansion of the boride is very similar to that for the carbide.

FIGS. 2 and 3 show the application of the invention to three-layer purification cells. In both figures a current-. conducting element 45, which is advantageously composed of one or more of the carbides and borides referred to, extends horizontally through the insulating (magnesite) wall 46 of the cell to project by its inner end into a depression 47 formed in the base of the cell so that, when the cell is in operation, it will be submerged in the body 48 of molten aluminum alloy which constitutes the bottom layer. The outer end of the element 45 is connected to an aluminum bus-bar 49 (which may be cast thereon) leading to the positive pole of the source of supply of the electrolyzin g current.

In FIG. 2 current-conducting elements 50 composed of one or more of the carbides and borides referred to constitute vertically disposed bars immersed by their lower ends in the layer 51 of purified aluminum floating on the body 52 of molten electrolyte, and attached at their upper ends to a negative bus-bar 53. These elements may be connected to the bus-bar by being brazed thereto (as at 54) or by having the bar 53 cast about their upper ends. The exposed portions of the elements 50 may be further protected against oxidation and other harmful effects by a sleeve 55 or, alternatively, the sleeve may be cast therearound as will be described hereinafter.

FIG. 3 shows a modified arrangement of currentconducting elements wherein the element supplying the top layer 51, and designated as 56, is arranged in substantially the same manner as the lead 45 for supplying the bottom layer 48, that is to say, it extends horizontally through the side Wall of the cell and is connected at its outer end to an aluminum bus-bar 57 which is, in this case, connected to the negative pole of the current-supply source.

With regard to three-layer purification or refining cells of the type mentioned above (and described with reference to FIGURE 7) it may be said that the insulating Wall 46 defines a chamber having an upper zone (horizontal) adapted to receive a body of molten purified aluminum (cathode), a lower zone (horizontal) adapted to receive a body of molten aluminum alloy (anode), and an intermediate zone adapted to hold a charge or body of molten flux or electrolyte.

As has been mentioned above it is desirable to protect the current-conducting elements against oxidation and corrosion where they are exposed for part of their length to oxidizing or corrosive atmospheres as, for example,

the current-conducting elements 50 illustrated in FIG. 2. This is particularly so when the elements, cathodes, or extensions thereof are formed from the carbides, as distinguished from the borides such as titanium boride, the carbides being subject to a particularly penetrating form of oxidation at temperatures of about 450 C. at which a powdery non-protective oxidation product is formed. A satisfactory method of protecting an element is to cover the exposed portion thereof with a sheath ofaluminum. However, the mere application of an aluminum cover fitted mechanically over the lead, or even cast around it does not afford adequate protection.

Although the carbides and borides referred to can be wetted with aluminum in a vacuum at a temperature about 1150 C. or electrolytically by making the material the coating on to the surface of the material, and then 'to cast on aluminum under conditions such that it will 'firmly adherent outer sheath of aluminum which affords very good protection from oxidation.

As an example, the separate steps in the process may be carried out as follows:

(a) A hot-pressed TiC bar 2" in diameter (to be used as a current-conducting element) is thoroughly cleaned to remove the graphitic surface layer. This is most readily carried out by sand-blasting, but chemical cleaning (for example, in a hot alkaline potassium permanganate solution) may also be used successfully. The bar must not be handled after cleaning.

(b) The bar is washed with water and immediately plated with cobalt or nickel to a thickness of about 0.001 in. An ammoniacal cobalt sulphate bath is suitable for the cobalt plating and a nickel sulphate ammonium chloride bath for the nickel plating.

(c) The plated bar is heated in a neutral or reducing atmosphere to approximately 1050 C. and maintained at that temperature for about 30 minutes to fire the cobalt 'or nickel onto the'carbide.v This treatment is conveniently carried out in an electric furnace provided with a hydrogen atmosphere, but good results have also been obtained by inserting the bar into a closed graphite container which delayed by local heating so that directional solidification of the metal towards the head is encouraged.

As a result a sound sheath A" thick is produced around the bar over practically the whole length thereof (one end beingleft bare) and continues as a solid rod (2%" in diameter) for approximately 10 ins. beyond the end of the TiC bar.

A complete sheathed electrode composed of titanium carbide and ready for insertion into a three-layer cell is shown in FIGS. 4 to 6. The TiC bar 59 is 2 ins. in diameter and is normally about 9 /2 ins. long; it may, however, be very much shorter than this if desired, and bars only 4 ins. long have been operated successfully.

The aluminum sheath 60 is conveniently of the thickness mentioned above, i.e. A inch, but this may be varied widely without affecting the efficient functioning of the electrode. The purity of aluminum used is not very important since only a small amount of it is finally dissolved in the cathode metal of the cell. Commercial purity (99.2% aluminum is quite suitable for the purpose.

During the operation of casting-on the sheath the bottom end 61 of the TiC bar is inserted into a graphite jig in order to centralize the bar in the mold. This portion of the bar (approximately /2 inch long) is thus not covered with aluminum. The electrode is normally immersed in-the metal layer in the cell, however, to a depth of about 2 inches, the level of the surface of this layer being indicated at 62 in FIG. 4, and it is important to ensure that the aluminum sheath is properly bonded to the TiC to below this level in the casting-on operation. When in use in a cell, the sheath 60 on the bar 59 melts back to a point about /2 inch above the level 62, and the skin of metal left behind by the adherent sheath serves to anaaeaa i protect the exposed interface from oxidation and attack by flux vapours.

Electrical connection from the external circuit is made to the end of the solid portion 63 of the sheath extending beyond the bar 59. This may be conveniently done by flattening the end 64 of the solid portion 63 and welding it directly to a suitable aluminum lead, e.g. a rectangular bar with a cross-section of 4 x inches as shown at 65.

With regard to the above manner of sheathing the end of current-conducting element 59, it is to be understood that such procedure can be used as an alternative to that hereinbefore set forth for afiixing the cathodic currentconducting elements of reduction cells to the bus-bars such as that shown for FIGURES 2 and 3.

In order to reduce losses of energy from the cell by radiation and convection, it is advantageous to use a screen over the layer 62 of cathode metal. This may be arranged to depend from the electrodes themselves and a type of screen that may be used is shown in FIG. 4. It consists of an aluminum plate 66 approximately A" thick, which fits over the sheath 60 and is supported by a collar 67; this can be adjusted to any given position and locked by means of the screws 63 which penetrate into the aluminum sheath.

One way in which electrodes of the type described above may be utilized in a three-layer cell is illustrated in FIG. 7.

The main structure of the cell consists of a base and outer container 79, constructed from refractory bricks of a material, such as magnesite, which is resistant to the molten electrolytes used in the electrolysis. Iron conductor bars '71 are inserted through the side wall and are supported by the base of the cell; they are left projecting on the exterior of the cell so that convenient connection may be made to the positive pole of the external electrical circuit. The internal floor of the cell is constructed of carbon or graphite blocks 72, which are fitted over the conductor bars and are in good electrical contact with them.

The molten bath itself consists of an anode layer 73 of aluminum containing a high proportion of copper to increase its density, an electrolyte layer 74, and a cathode layer 62 of purified aluminum. The temperature of the bath under normal running conditions is usually in the range 740-7S0 C.

Electrical connection is made to the cathode layer 62 by means of the sheathed electrodes described above. The TiC bars 59 are immersed in the molten aluminum to a depth of about 2 inches. The aluminum conductor 65, which forms the main suspension of each electrode, is clamped to the main external electrical bus-bar or cathode beam 75. These beams are normally arranged so that they can be adjusted in height; thus when the level of the molten bath changes (for example, when the pure metal in the top layer is tapped oif) all the electrodes can be raised or lowered simultaneously to correct their immersion. Individual adjustment of each electrode can be made by means of the clamp '76 used to hold the electrode in good electrical contact with the beam.

Tests made on a cell constructed in the manner illustrated in FIG. 7 embodying current-conducting elements 59 of titanium carbide illustrate the advantages of such a cell as compared to a corresponding orthodox cell. The orthodox cell embodied nine inch diameter electrodes each carrying a current of 3,000 amperes and wherein the voltage drop from the conductor 65 to the purified aluminum layer 62 was 0.4 to 0.6 volts. The modified cell embodied eighteen 2 inch diameter electrodes each carrying a current of 1500 amperes and wherein the volt-age drop from the conductor 65 to the purified aluminum layer 62 was 0.02 to 0.08 volts. The carbide elements required for the full equipment of a cell were very much less bulky than the corresponding graphite electrodes due to their lower resistivity and this allows the electrodes to be arranged to considerable 15 advantage with respect to shielding of the upper surface of the bath to minimize heat losses, and this aspect is of great importance in overall etficiency of operation.

In order to minimize the loss of energy from the cell by radiation and convection, it is beneficial to cover in the top with appropriate screens, This can be done by a suitable combination of screens 66 carried by the individual electrodes, and other screens 77 which rest on the side walls of the cell.

It should be noted that the 'IiC bars are liable to crack if subjected to severe thermal shock, and it is advisable to introduce them into the cell slowly, so that they are pre-heated. before entering the molten aluminum layer. Alternatively, a thermal barrier is provided by coating the bars with tar and then dusting them with carbon black. This delays the heating of the bars so that they may be inserted directly in the layer of molten aluminum without any risk of cracking.

Although the above description refers to current-conducting elements composed of titanium carbide, it will be appreciated that elements composed of the other carbides and the borides referred to a'bove may be sheathed with aluminum in a similar manner.

With further regard to the three-layer cells shown in FIGURES 2, 3 and 7, and described above, it is also contemplated within the scope of the invention that electrical connection with the bottom layer of molten metal, i.e. the molten aluminum alloy layer, can be by provision of one or more current-conducting elements projecting upwardly from the bottom of the cell, rather than by means of elements extending horizontally through the insulating Wall or use of carbon or graphite blocks as the cell floor.

It will be seen that the modification of electrolytic A context, i.e. the use of the carbides and borides of titanium, zirconium, tantalum and niobium and mixtures thereof as cathodes and leads to operate in contact with both molten aluminum-containing and molten electrolyte in such electrolytic cells. From the data given above, it will be seen that although these advantages were first achieved by the use of the carbides of these elements, the borides were subsequently found to be markedly superior in many respects and can be used more beneficially in most of the structures cited.

The boride and carbide materials mentioned above have been referred to by certain text writers as refractory hard metals. See Schwarzkopf et al. Refractory Hard Metals published by The MacMillan Company, 1953.

It will be obvious that various modifications and alterations may be made in this invention without departing from the spirit and scope thereof and it is not to be taken as limited except by the dependent claims herein.

What is claimed is:'

1. A method of producing a current-conducting element which comprises electro-plating with cobalt in an ammoniacal cobalt sulphate bath a bar consisting essentially of at least one of the materials selected from the group consisting of the carbides of titanium, zirconium, tantalum and niobium, sintering the plated coating onto the surface of the bar-and then casting an aluminum sheath onto the plated bar under conditions such that it will alloy with the plated coating.

2. A method of producing a current-conducting ele ment which comprises electro-plating with cobalt in an ammoniacal cobalt sulphate bath a bar which consists essentially of at least one of the materials selected from the group consisting of the borides of titanium, zirconium, tantalum and niobium, sintering the plated coating onto the surface of the bar and then casting an aluminum sheath onto the plated bar under conditions such that it will alloy with the plated coating.

3. A method of producing a current-conducting element which comprises electro-plating with nickel in a nickel sulpate ammonium chloride bath a bar consisting essentially of at least one of the carbides from the groups consisting of carbides of titanium, zirconium, tantalum and niobium, sintering the plated coating onto the surface of the bar and then casting an aluminum sheath onto the plated bar under conditions such that it will alloy with the plated coating.

4. A method of producing a current-conducting element which comprises electro-plating with nickel in a nickel sulphate ammonium chloride bath a bar consisting essentially of at least one of the borides from the group consisting of borides of titanium, zirconium, tantalum and niobium, sintering the plated coating onto the surtace of the bar and then casting an aluminum sheath onto the plated bar under conditions such that it will alloy with the plated coating.

5. A method of producing a current-conducting element which comprises electroplating with one of the materials from the group consisting of cobalt and nickel a bar which consists essentially of at least one of the carbides from the group consisting of carbides of titanium, zirconium, tantalum and niobium, heating the plated bar in a non-oxidizing atmosphere to a temperature of approximately 105 C. and maintaining it at that temperature for about 30 minutes to fire the plated coating onto the bar, and then casting an aluminum sheath onto the plated bar under conditions such that it will alloy with the plated coating.

6. A method according to claim 5 wherein the heat treatment of the plated bar is carried out in an electric furnace provided with a hydrogen atmosphere.

7. A method according to claim 5 wherein the heat treatment of the plated bar is carried out in a closed graphite container.

8. A method of producing a current-conducting element which comprises electro-plating with one of the materials from the group consisting of cobalt and nickel a bar consisting essentially of at least one of the carbides from the groups consisting of carbides of titanium, zirconium, tantalum and niobium, sintering the plated coating onto the surface of the bar, setting the plated bar in a mold, heating the plated bar to a temperature of from 750 to 800 C., pouring molten aluminum at substantially the same temperature into the mold and after a brief period at this temperature allowing the mold to cool.

9. A method of producing a current-conducting element which comprises electro-plating with one of the materials from the group consisting of cobalt and nickel a bar consisting essentially of at least one of the carbides from the group consisting of carbides of titanium, zirconium, tantalum and niobium, heating the plated bar in a non-oxidizing atmosphere to a temperature of appr0ximately 1050 C. and maintaining it at that temperature for about 30 minutes to fire the plated coating onto the bar, setting the plated bar within a graphite mold with its bottom end inserted in a graphite jig in order to centralize the bar in the mold, heating the plated bar and the mold to a temperature of 750 to 800 C., pouring molten aluminum at substantially the same temperature into the mold through a large feeding head, and allowing the mold to cool after a brief period at this temperature whilst delaying cooling of said headrby localized heating to encourage directional solidification ofthe metal towards said head.

10..A method of producing a current-conducting element which comprises electro-plating with one of the materials from the groups consisting of cobalt and nickel a bar consisting essentially of at least one of the borides from the group consisting of borides of titanium, zirconium, tantalum and niobium, heating the plated bar in a non-oxidizing atmosphere'to a temperature'of approximately 1050 C. and maintaining it at that temperature for about 30 minutes to fire the plated coating onto the bar, setting the plated bar within a graphite mold with its to a temperature of 750 to 800 C., pouring molten aluminum at substantially the same temperature into the mold through a large feeding head, and allowing the mold to cool after, a brief period at this temperature whilst delaying cooling of said head by localized heating to encourage directional solidification of the metal to wards said head.

11. A method of producing a current-conducting element which comprises electro-plating with one of the materials from the group consisting of cobalt and nickel a bar consisting essentially of a mixture of at least one of the carbides and at least one of the borides from the group consisting of carbides and borides of titanium, zirconium, tantalum and niobium, heating the plated bar in a non-oxidizing atmosphere to a temperature of approximately 0 C. and maintaining it at that temperature for about 30 minutes to fire the plated coating onto the bar, setting the plated bar within a graphite mold with its'bottom end inserted in a graphite jig in order to centralize the bar in the mold, heating the plated bar and the mold to a temperature of 750 to 800 C., pouring molten aluminum at substantially the same temperature into the mold through a large feeding head, and allowing the mold to cool after a brief period at this temperature whilst delaying cooling of said head by localized heating to encourage directional solidification of the metal towards said head.

12. A method of producing a current conducting element which comprises electroplating with cobalt in an ammonical cobalt sulphate bath a bar consisting essentially of a mixture of at least one of the carbides and at least one of the borides from the group consisting of carbides and borides of titanium, zirconium, tantalum and niobium, sintering the plated coating onto the surface of the bar, and then casting an aluminum sheath onto the plated bar under conditions such that it will alloy with the plated coating.

13. A method of producing a current conducting element which comprises coating with a firmly adherent film of metal, a bar consisting essentially of at least one of the chemical compounds from the group consisting of the carbides of titanium, zirconium, tantalum, and niobium in a manner to bond said film to said bar and then casting an aluminum sheath onto said bar under conditions such that said sheath will alloy with said metal film.

14. A method of producing a current conducting element which comprises coating with a firmly adherent film of metal, a bar consisting essentially of at least one of the chemical compounds from the group consisting of the borides of titanium, zirconium, tantalum and niobium in a manner to bond said film to said bar and then casting 'an aluminum sheath onto said bar under conditions such that said sheath will alloy with said metal film.

15. A method of'producing a current conducting element which comprises coating with a firmly adherent film of metal, a bar consisting essentially of a mixture of at least one of the chemical compounds from the group con sisting of carbides and borides of titanium, tantalum, zirconium and niobium in a manner to bond said film to said bar and then casting an aluminum sheath onto said bar under conditions such that said sheath will alloy with said metal film.

16. A method of producing a current conducting element which comprises providing a coating of one of the materials from the group consisting of cobalt and nickel on a bar consisting essentially of at least one of the chemical compounds from the group consisting of borides and carbides of titanium, zirconium, tantalum and niobium so that said coating is bonded to said bar, heating the coating on the surface of the bar and then casting an aluminum sheath onto the coated bar under conditions such that it will alloy with the coating.

17. A method of producing a current conducting element which comprises providing a coating of one of the materials from the group consisting of cobalt and nickel on a bar consisting essentially of a mixture of at least one of the chemical compounds from the group consisting of carbides and borides of titanium, zirconium, tantalum and niobium so that said coating is bonded to said bar, heating the coating on the surface of the bar and then casting an aluminum sheath onto the coated bar under conditions such that it will alloy with the coating.

References Cited by the Examiner UNITED STATES PATENTS 1,760,586

5/30 Devers 22202 Comstock 22202 Harris et a1. 22202 McNab 22202 Whitfield et a1. 22204 Cottrell 22202 Reynolds 22204 Shaefer et al. 22203 XR Eklund 22204 Ellis et al.

Kruse 22203 XR MICHAEL V. BRINDISI, Primary Examiner.

CHARLES W. LANHAM, NEDWIN BERGER, DAVID 1,799,991 4 31 s n et a1, 22.403 15 RECK, MARCUS LYONS, Examlllers- 

15. A METHOD OF PRODUCING A CURRENT CONDUCTING ELEMENT WHICH COMPRISES COATING WITH A FIRMLY ADHERENT FILM OF METAL, A BAR CONSISTING ESSENTIALLY OF A MIXTURE OF AT LEAST ONE OF THE CHEMICAL COMPOUNDS FROM THE GROUP CONSISTING OF CARBIDES AND BORIDES OF TITANIUM, TANTALUM, ZIRCONIUM AND NIOBIUM IN A MANNER TO BOND SAID FILMS TO SAID BAR AND THEN CASTING AN ALUMINUM SHEATH ONTO SAID BAR UNDER CONDITIONS SUCH THAT SAID SHEATH WILL ALLOY WITH SAID METAL FILM. 